Scott Dunn is a serial killer and aspiring writer, who had perfected a method of combining literature with reality by committing murder sprees and then writing about them as fiction.

Biography

He once attended a book signing by Richard Castle, where Castle advised him to 'write what he knew' in his work (it was unspecified if this inspired Dunn's decision to commit murder sprees so that he could write about them, or if he had been doing this already, and Castle just 'confirmed' the merit of his 'strategy').

This psychosis came to a head when he decided to target Detective 'Nikki Heat' - in reality Detective Kate Beckett - his profile suggesting that he went after her as his perfect 'counterpart', representing a detective who was part-fiction and part-reality.

Having selected an unwitting 'fall guy' for the murder of his three victims - choosing a man whose dog had recently been killed by another dog; Dunn's victims being a lawyer that his victim had asked to represent him, the dog-walker who'd been walking the dog that killed his, and the taxidermist who'd been hired to stuff his pet - Dunn left a message for 'Nikki' in the bullets he put in his victims (Five bullets in his first victim, four in his second and four in his third, with the bullets presenting the words 'NIKKI' 'WILL' 'BURN' when lined up together).

Although the FBI team tracking Dunn's patsy believed that he had committed suicide when about to be caught, Castle realised the truth when he noted that their killer - who had briefly spoken to them over the phone while standing in the window of an apartment - was left-handed, yet the gun had been in the dead man's right hand. This epiphany gave him time to call Beckett and warn her before Dunn could set off a bomb in her apartment.

When Dunn realized that Beckett was still alive, he took out his rage by killing a woman at random. From his actions the FBI were able to deduce his true identity, after Beckett chased him through the subway. Although Dunn attempted to regain control of the situation by capturing FBI profiler Jordan Shaw, and pretending to provide the team with evidence that would allow them to find Shaw to lead them into a trap, his deception was deduced by Beckett and Castle, who realized that he was actually in the building opposite the one currently holding the FBI team, intending to lure them in and then blow the building up.

With no time to alert the FBI, Beckett and Castle entered the building themselves, successfully capturing Dunn and freeing Shaw. Shaw reflected as she departed that, in a sense, Dunn had been defeated by Nikki Heat, as Castle and Beckett - Heat's creator and her inspiration respectively - were essentially Nikki Heat when combined into one.